In many applications of logic circuits, it is essential that the logic circuit have a suitable fanout (ability to drive a large number loads) or equivalent capacitive drive capability. In order to provide sufficient current drive to heavy capacitive loads, an output buffer is coupled between the output of the logic circuit and the load which is to be driven. Conventional output driver circuits comprise a transistor amplifier, usually in the form of an emitter follower, which has a low output impedance, thereby providing high drive capability, reduced delay per unit load, and fast switching speed. BICMOS circuits combine CMOS devices having high input impedance, good noise immunity and low static power dissipation, and bipolar output transistors having low output impedance and excellent current drive capability.
Monolithically integrated CMOS and BICMOS circuits typically operate with a power supply voltage of 5.0 volts. In the case of BICMOS circuits, the bipolar output device is limited to a range of output voltage swing approximated by the power supply voltage minus two base-emitter voltages. CMOS circuits provide output voltage levels equal to the power supply voltage; however, the output impedance is quite high and the current drive is reduced.
In an effort to avoid reliability problems associated with high electric field in the gate-drain region (hot electron effect), CMOS and BICMOS circuits having submicron dimensions typically require a voltage supply not to exceed 3.4 volts. As a consequence, systems designed to operate from a 5.0 volt power supply require modification to accommodate a CMOS or BICMOS circuit with submicron dimensions.
Thus, what is needed is a BICMOS driver circuit having high input impedance and high output current drive that provides the supply voltage and full logic output voltage swing for the submicron circuitry.